A battery-driven device, such as a smart phone, a mobile phone, a tablet PC (Personal Computer), a laptop PC, and a portable audio player, may include a rechargeable secondary battery and a charging circuit for charging the battery. From among various charging circuits, there is a charging circuit that charges a secondary battery based on a DC bus voltage supplied from a USB host adapter via a USB cable.
Currently, charging circuits included in mobile devices comply with the USB Battery Charging Specification (hereinafter referred to as BC specification) standard. Several types of host adapters are currently available. The BC revision 1.2 specification defines types of chargers including a SDP (Standard Downstream Port), a DCP (Dedicated Charging Port), and a CDP (Charging Downstream Port). Furthermore, currents (current capacities) that can be supplied by host adapters are defined according to types of chargers. Specifically, a current of 1500 mA is defined for the DCP and the CDP, while currents of 100 mA, 500 mA, and 900 mA are defined for the SDP according to versions of USB.
As a next-generation method and system for charging a secondary battery using USB, a specification called as USB Power Delivery (hereinafter referred to as PD specification) has been introduced. In the PD specification, a suppliable power is significantly increased to a maximum of 100 W from 7.5 W of the BC specification. Specifically, in the PD specification, a voltage higher than 5V (e.g. 12V or 20V) is allowed to be supplied as a USB bus voltage, and a supply of a charging current (e.g., 2 A, 3 A, and 5 A) greater than that in the BC specification is also allowed.
A power supply circuit that generates a bus voltage and a USB-PD controller that communicates with a power receiving device via a bus line are included in a power supply device compliant with a USB-PD specification. A USB-PD controller communicates with a power receiving device and controls a power supply circuit by negotiating a voltage level of a bus voltage. A control sequence of such a USB-PD controller is managed by a processor that executes programs.
To monitor congestion of a processor and reset a system in case of a malfunction, a watchdog timer (WDT) is used. A program executed by a processor is written to reset the WDT in a predetermined reset cycle, e.g., once every a few seconds. An overflow cycle of the WDT is set to be longer than a reset cycle of the WDT.
While a processor normally operates, a WDT is reset in a predetermined reset cycle before the WDT overflows. Meanwhile, if congestion occurs at a processor due to any type of malfunction, the WDT is not reset but overflows, and thus the malfunction may be detected.
A system becomes out of control for a few seconds from the malfunction of a processor until the WDT detects the malfunction of the processor. If a system goes out of control in a power supply device, a high voltage exceeding a breakdown voltage of a power receiving device may be output for a few seconds and may cause adverse effects to the reliability of the power receiving device or the power supply device itself.